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The Role of Imaging in IMRTThe Role of Imaging in IMRT
James F. Dempsey, Ph.D.James F. Dempsey, Ph.D.Department of Radiation OncologyDepartment of Radiation Oncology
University of Florida College of MedicineUniversity of Florida College of MedicineGainesville, Florida USAGainesville, Florida USA
AcknowledgementsAcknowledgements
nn Omar A. Zeidan, Ph.D.Omar A. Zeidan, Ph.D. Univ. of FloridaUniv. of Floridann Tony Tony StellStell Univ. of FloridaUniv. of Floridann ChihrayChihray Liu, Ph.D.Liu, Ph.D. Univ. of FloridaUniv. of Floridann Jonathan G. Li, Ph.D.Jonathan G. Li, Ph.D. Univ. of FloridaUniv. of Floridann Jatinder R. Palta, Ph.D. Jatinder R. Palta, Ph.D. Univ. of FloridaUniv. of Floridann Daniel A. Low, Ph.D.Daniel A. Low, Ph.D. Washington UniversityWashington Universitynn Clifford K.S. Chao, M.D.Clifford K.S. Chao, M.D. M.D. Anderson M.D. Anderson nn Jeffrey F. Williamson, Ph.D.Jeffrey F. Williamson, Ph.D. Washington UniversityWashington Universitynn Sasa Mutic, M.S.Sasa Mutic, M.S. Washington UniversityWashington Universitynn Robert Robert MalyapaMalyapa, M.D., Ph.D. , M.D., Ph.D. Washington UniversityWashington Universitynn Perry W. Grigsby, M.D.Perry W. Grigsby, M.D. Washington UniversityWashington University
OutlineOutlinenn Why IMRT Needs Quantitative ImagingWhy IMRT Needs Quantitative Imagingnn The “Theory” of Quantitative ImagingThe “Theory” of Quantitative Imaging
nn Sampling & ReconstructionSampling & Reconstructionnn Filters for Linear SystemsFilters for Linear Systems
nn Survey of the Roles of Quantitative Imaging in Survey of the Roles of Quantitative Imaging in IMRTIMRTnn Imaging for Simulation & Treatment PlanningImaging for Simulation & Treatment Planningnn Imaging for Target Delineation Imaging for Target Delineation nn Imaging for Dose MeasurementImaging for Dose Measurementnn Imaging for Delivery ValidationImaging for Delivery Validation
nn Methods of Delivery Validation via ImagingMethods of Delivery Validation via Imaging
Clinical Motivation: Clinical Motivation: ConformalityConformality => Better Sparing=> Better Sparing
Screen Capture from Nomos Peacock Tx Planning System MIR Washington Univ.
Parotid Sparing in the Presence of Parotid Sparing in the Presence of Systematic Setup ErrorSystematic Setup Error
0 1 0 20 30 40 50 60 7 0 800
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1Effect of Lateral Shift on Left Parotid Dose
Fra
ctio
n V
olum
e
Dose (Gy)
15 mm
10 mm
5 mmCorrect But Random Set-
Up Errors will “wash out” any overdosing of the parotid, right?
Systematic Set-up errors can have a significant impact on critical structure sparing
Parotid Sparing in the Presence of Parotid Sparing in the Presence of Random SetRandom Set--Up ErrorUp Error
Well No,
With IMRT we have purposely placed the parotid in a high gradient
2
Questions for IMRTQuestions for IMRT
nn How Do We Ensure Our IMRT Dose Calculations How Do We Ensure Our IMRT Dose Calculations are Accurate?are Accurate?
nn How Do We Ensure That Can Achieve Our How Do We Ensure That Can Achieve Our Clinical Goals?Clinical Goals?
nn How can we best employ IMRT to the greatest How can we best employ IMRT to the greatest efficacy?efficacy?
Could Quantitative Imaging Be the Answer?Could Quantitative Imaging Be the Answer?
What is Quantitative Imaging?!What is Quantitative Imaging?!
nn An Oxymoron?...An Oxymoron?...nn Most “Imaging Science” is concerned with Most “Imaging Science” is concerned with
Qualitative Feature Extraction Qualitative Feature Extraction nn e.g. for diagnosise.g. for diagnosis
nn Radiation Therapy Attempts to Perform Radiation Therapy Attempts to Perform QuantitativeQuantitative Measurements Measurements Using Using Imaging TechniquesImaging Techniques
nn Lets Look at the applications for IMRTLets Look at the applications for IMRT
Where Do We Need Quantitative Where Do We Need Quantitative Imaging Techniques in IMRT?Imaging Techniques in IMRT?
nn Imaging for Simulation & Treatment PlanningImaging for Simulation & Treatment Planningnn CT based Heterogeneity CorrectionsCT based Heterogeneity Correctionsnn BreathBreath--Hold Gated CTHold Gated CTnn CT or MR Cine’ for Organ MotionCT or MR Cine’ for Organ Motion
nn Imaging for Target Delineation Imaging for Target Delineation nn Pet Registration & FusionPet Registration & Fusionnn MR Registration & FusionMR Registration & Fusion
nn Imaging for Delivery ValidationImaging for Delivery Validationnn Fluence Map ValidationFluence Map Validation
nn Imaging for Dose MeasurementImaging for Dose Measurementnn Film DosimetryFilm Dosimetrynn Gel DosimetryGel Dosimetrynn Exit DosimetryExit Dosimetry
Theory of “Quantitative” ImagingTheory of “Quantitative” Imaging
nn All of these applications attempt to extract All of these applications attempt to extract quantitative information from imaging quantitative information from imaging
nn We must 1We must 1stst ask, how do we know that ask, how do we know that the imaging devices are capable of the imaging devices are capable of measuring the distributions that we seek?measuring the distributions that we seek?
nn Secondly, If the device fails to accurately Secondly, If the device fails to accurately measure our distribution can we correct or measure our distribution can we correct or recover the information?recover the information?
How to Perform Quantitative How to Perform Quantitative ImagingImaging
nn 1) Sample Data with High Enough Frequency1) Sample Data with High Enough Frequencynn 2) Characterize the Linearity and Spatial 2) Characterize the Linearity and Spatial
Independence of the imaging systemIndependence of the imaging systemnn 3) Determine Line Spread and or Point Spread 3) Determine Line Spread and or Point Spread
functions and Modulation Transfer Functionsfunctions and Modulation Transfer Functionsnn 4) Evaluate the Ability to Make Quantitative 4) Evaluate the Ability to Make Quantitative
MeasurementsMeasurementsnn 5) Apply Filters to Recover Information if 5) Apply Filters to Recover Information if
necessarynecessary
ShannonShannon--NyquistNyquist Sampling Sampling Theorem (1)Theorem (1)
nn If a function has all of its frequency components If a function has all of its frequency components below some frequency n, then sampling that below some frequency n, then sampling that function with frequency 2function with frequency 2νν allows for the exact allows for the exact reconstruction of that functionreconstruction of that function
F(x) F(ω)
ν
ω [freq.]x [dist]
3
ShannonShannon--NyquistNyquist Sampling Sampling Theorem (1)Theorem (1)
nn With this theorem and your knowledge of With this theorem and your knowledge of Medical Physics you can determine the temporal Medical Physics you can determine the temporal and spatial resolutions at which distributions and spatial resolutions at which distributions should be sampled to prevent aliasing!should be sampled to prevent aliasing!
nn For example, if we know a priori that a dose For example, if we know a priori that a dose distribution takes on a functional form we can distribution takes on a functional form we can determine the required sampling as follows …determine the required sampling as follows …
Example of the Use of Sampling Example of the Use of Sampling Theory (1)Theory (1)
A fitted 1x1 cm2 profile from 0.1 mm pixel RCF measurements in solid water
Practical Sampling Theory (1)Practical Sampling Theory (1) Linear Systems Theory (2)Linear Systems Theory (2)
nn A system (a transformation T) is considered A system (a transformation T) is considered “linear” if it have the following properties:“linear” if it have the following properties:
( )[ ] ( ) )()(^
ϖϖ GbFaxgbxfa ×+×=×+×Τ
In other words, superposition and scaling hold under the transformation
Here a system is though to be an imaging system of course!
Linear Systems and Imaging (2)Linear Systems and Imaging (2)
nn If an imaging system can be approximated If an imaging system can be approximated as a linear system …as a linear system …
nn And any process that degrades the output And any process that degrades the output of the imager is a linear process …of the imager is a linear process …
nn And the degrading process is spatially or And the degrading process is spatially or temporally invariant …temporally invariant …
nn Then… Then…
Spread Functions (3)Spread Functions (3)
nn Convolution with Spread Functions can be used Convolution with Spread Functions can be used to characterize the degradation of the imaging to characterize the degradation of the imaging data.data.
True Signal Imaged Signal
Spread Function
)()()( xtionSpreadFucnxTrueSignalxSignal ⊗=
4
Spread Functions (3)Spread Functions (3)
nn Spread functions plot the redistribution of data Spread functions plot the redistribution of data in a pixel due to a in a pixel due to a degradatingdegradating processprocess
nn Line Spread Functions (LSF) characterize 1D Line Spread Functions (LSF) characterize 1D processprocess
nn Point Spread Functions (PSF) characterize 2D (or Point Spread Functions (PSF) characterize 2D (or 3D) process3D) process
nn If a PSF is isotropic it can be related to the LSF If a PSF is isotropic it can be related to the LSF by the Abel Transformby the Abel Transform
Modulation Transfer Function Modulation Transfer Function (MTF) (4)(MTF) (4)
The Fourier Transform of the LSF or the HankelTransform of the PSF produce the MTF: The MTF needs to be very close to a value of unity for quantitative systems !
The Fourier Convolution Theorem The Fourier Convolution Theorem Can Be Used to Recover Data (5)Can Be Used to Recover Data (5)
)(/)()(~~~
ϖϖϖ tionSpreadFucnSignalTrueSignal =
Quantitative Imaging SummaryQuantitative Imaging Summary
nn 1) Sample Data with High Enough Frequency1) Sample Data with High Enough Frequencynn 2) Characterize the Linearity and Spatial 2) Characterize the Linearity and Spatial
Independence of the imaging systemIndependence of the imaging systemnn 3) Determine Line Spread and or Point Spread 3) Determine Line Spread and or Point Spread
functions and Modulation Transfer Functionsfunctions and Modulation Transfer Functionsnn 4) Evaluate the Ability to Make Quantitative 4) Evaluate the Ability to Make Quantitative
MeasurementsMeasurementsnn 5) Apply Filters to Recover Information if 5) Apply Filters to Recover Information if
necessarynecessary
Imaging for Simulation & Imaging for Simulation & Treatment PlanningTreatment Planning
Brief Survey of Imaging in IMRTBrief Survey of Imaging in IMRTVirtual simulation for Virtual simulation for IMRT Using XIMRT Using X--Ray CTRay CT
nn Due to it’s inherent Due to it’s inherent spatial integrity Xspatial integrity X--ray CT remains the ray CT remains the “Gold Standard” “Gold Standard” for Tx Planningfor Tx Planning
nn CT Data can allow CT Data can allow dose calculations dose calculations to account for to account for Tissue Tissue Heterogeneities Heterogeneities
nn We are relying We are relying on accurate on accurate CT#sCT#s
5
Heterogeneities in IMRTHeterogeneities in IMRT
Heterogeneities can cause large discrepancies in the presences of air cavities for IMRT.
Shown: CCC w/ and w/o heterogeneities for nasopharyngeal target
CT ArtifactsCT Artifacts•Artifactual CT Numbers can be produced in the presence of object that can
•Spatial Integrity of the Data is not compromised
•Assigning Bulk Densities to Structures can overcome Heterogeneity corrections to Artifactual CT Number regions
•IEEE Trans Med. Imaging 2001 20(10) 1009-1017 Snyder and Williamson et al.
High Quality DRR and Portal Films High Quality DRR and Portal Films are Critical to IMRT are Critical to IMRT
Port Films are required to verify isocenter to within a few mm
Spatial Integrity is of Utmost Importance: Feature Extraction is qualitative
Serial Tomotherapy DRR and Port Film
Adaptive Radiotherapy & IMRTAdaptive Radiotherapy & IMRTXX--Ray Cone Beam Reconstruction Ray Cone Beam Reconstruction Cone Beam Imaging at The Accelerator can Provide Daily Setup Based on the Imaging of Soft Tissues!
DA Jaffray, DG Drake, M Moreau, AA Martinez, and JW. Wong, Int. J. Radiation Oncology Biol. Phys., Vol. 45, No. 3, pp. 773–789, 1999
Adaptive Radiotherapy & IMRTAdaptive Radiotherapy & IMRTMegaVoltageMegaVoltage CTCT
Helical Tomotherapy and Megavoltage CT Imaging at The Accelerator
K J Ruchala, G H Olivera , E A Schloesser and T R Mackie
Phys. Med. Biol. 44 (1999) 2597–2621.
Imaging for Target DelineationImaging for Target Delineation
Brief Survey of Imaging in IMRTBrief Survey of Imaging in IMRT
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Virtual Simulation Software QAVirtual Simulation Software QAMULTIMODALITY IMAGE REGISTRATIONMULTIMODALITY IMAGE REGISTRATION & FUSION& FUSION
nn Validation of the Validation of the Complete Clinical Complete Clinical Process is Important:Process is Important:
nn Mutic et al. Mutic et al. Multimodality image Multimodality image registration quality registration quality assurance for assurance for conformal threeconformal three--dimensional treatment dimensional treatment planning. Int J Radiat planning. Int J Radiat Oncol Biol Phys, 2001Oncol Biol Phys, 2001
a) b)
c) d)
MR PET
CT2CT1
MRIMRI--CT Image FusionCT Image Fusion--RegistrationRegistration
•• Magnetic Resonance Imaging (MRI)Magnetic Resonance Imaging (MRI)
•• Excellent soft tissue contrast allows Excellent soft tissue contrast allows better differentiation between better differentiation between normal tissues and many tumorsnormal tissues and many tumors
•• It is not limited to imaging in axial It is not limited to imaging in axial planes planes
•• Disadvantages:Disadvantages:
–– Susceptible to spatial Susceptible to spatial distortionsdistortions
–– Image intensity values Image intensity values do not relate to physical do not relate to physical or electron densityor electron density
MR
CT
CUCU--ATSM PETATSM PET--CT Image Registration & CT Image Registration & Target DefinitionTarget Definition
Chao et al. Int J Radiat Oncol Biol Phys 2001 Mar 15;49(4):1171-82
FDG-PET Scan CT Scan
Para-aortic L.N.
Kidney
Target Delineation By PET – CT Registration & Target DefinitionRegistration & Target Definition
R. Malyapa et al. MO-D-517A-07
Imaging for Lung IMRT?Imaging for Lung IMRT?
nn Attractive IMRT Site Due to Need for Attractive IMRT Site Due to Need for Conformal Dose Distributions and Target Conformal Dose Distributions and Target Encompassing Critical StructuresEncompassing Critical Structures
nn Significant Concern Over Loss of Significant Concern Over Loss of Superposition Due to Breathing Motion Superposition Due to Breathing Motion and Significant Heterogeneity Correctionsand Significant Heterogeneity Corrections
Spirometer Gated Spirometer Gated MultisliceMultislice CTCT
D. Low et al. WED-C-517D
Multi -slice CT gated on tidal lung volume as measured by digital spirometry during free breathing
Reconstruction is performed using sinograms that are assembled by the coincident spirometer reading
Produces time dependent CT data to map out breathing motions
7
Imaging for Dose MeasurementImaging for Dose Measurement
Brief Survey of Imaging in IMRTBrief Survey of Imaging in IMRTRadiographic Film Dosimetry for Radiographic Film Dosimetry for
Patient Specific QAPatient Specific QA
Radiochromic Film Dosimetry for Radiochromic Film Dosimetry for Commissioning IMRT Treatment Commissioning IMRT Treatment
Planning SystemsPlanning Systems600
900
1200
1400
300
Polymerizing Gel Dosimetry in Polymerizing Gel Dosimetry in IMRTIMRT
Imaging for IMRT Delivery Imaging for IMRT Delivery ValidationValidation
nn How Do We Validate the Complex How Do We Validate the Complex Orchestration of MLC Leaf Motion and Orchestration of MLC Leaf Motion and Dose Delivery That Occurs in SMLCDose Delivery That Occurs in SMLC-- and and DMLCDMLC--Based IMRT?Based IMRT?
nn Imaging of the integral dose or fluence Imaging of the integral dose or fluence with film or EPIDwith film or EPID
nn Time Resolved Imaging of the Fluence or Time Resolved Imaging of the Fluence or DoseDose
MLC Delivery Device MLC Delivery Device Characterization via FilmCharacterization via Film
•Radiographic meas. of accuracy of leaf travel/ offsetAnd Integral Fluence Maps
LoSasso T et al. Med. Phys. 2001; 28(11):2209-2219.
8
EPID and IMRTEPID and IMRTDelivery Validation Delivery Validation
EPID Records Integral or Time Resolved Exit Dose to Validate Fluence Map Delivery: Most Time Resolved Systems <10 FPS
Partridge M et al., Med. Phys. 2000; 27(7):1601-1609.
Accelerator Log Files ~20 FPS Accelerator Log Files ~20 FPS (~50 ms sample rate)(~50 ms sample rate)
LoSasso T et al. Med. Phys. 2001; 28(11):2209-2219.
Xia P et al. Med. Phys. 2002; 29(3):412-423.
Ezzell and Chungbin J Appl Clin Med Phys 2001; 2(3):138-148.
Litzenberg et al. J Appl Clin Med Phys 2002; 3(2):63-72.
ScintillationScintillation--CCD Camera Validation CCD Camera Validation of MLC Deliveryof MLC Delivery
ScintillationScintillation--CCD CameraCCD CameraCharacterizationCharacterization
nn Fast Gd2O2S:Tb Scintillation Plate coupled to a Fast Gd2O2S:Tb Scintillation Plate coupled to a CCD or CMOS Camera for high frame speed CCD or CMOS Camera for high frame speed capture.capture.
n Only 1.24 g/cm2 stainless steel, 0.411 g/cm2 Gd2O2S:Tb, and 0.008 g/cm2 aluminized mylarin the beam path: allows verifcation upstream of the patient
nn CCD Camera up to 30 FPSCCD Camera up to 30 FPSnn CMOS Camera up to 1000 FPS (200 FPS is good CMOS Camera up to 1000 FPS (200 FPS is good
enough)enough)
ScintillationScintillation--CCD CameraCCD CameraCharacterizationCharacterization
nn Sampling in Time & SpaceSampling in Time & Spacenn Demonstration of Linearity as well as Demonstration of Linearity as well as
Spatial and Temporal Invariance Spatial and Temporal Invariance nn Spatial PSFs,Temporal Spatial PSFs,Temporal LSFsLSFs, and , and
associated MTFsassociated MTFs
Scintillation Camera Validation of Scintillation Camera Validation of IMRT DeliveryIMRT Delivery
Gd2O2S:TB Scintillation plate
Camera:
30 fps CCD Camera
1000 fps CMOS Camera
Observation of Leaf Motion During SMLC Delivery
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30 FPS30 FPS Comparison of Log Files, Camera, Comparison of Log Files, Camera, and the Intended Deliveryand the Intended Delivery
100 FPS100 FPS Much Ado About Nothing?Much Ado About Nothing?
Feedback Treatment Planning Feedback Treatment Planning
nn Imaged MLC Delivery Errors are accounted Imaged MLC Delivery Errors are accounted for in a recalculated Treatment Planfor in a recalculated Treatment Plan
What Should I Do when What Should I Do when Quantitative Imaging is used In My Quantitative Imaging is used In My
ClinicClinicnn Understand the IssuesUnderstand the Issuesnn Understand the TheoryUnderstand the Theorynn Insist that vendors of a device that is intended Insist that vendors of a device that is intended
for quantitative imaging measurement for quantitative imaging measurement demonstratedemonstratenn LinearityLinearitynn LSFsLSFs, , PSFsPSFs, and MTFs where appropriate, and MTFs where appropriate
nn Use your knowledge of dosimetry to evaluate Use your knowledge of dosimetry to evaluate the practical limits of the device the practical limits of the device
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One should ask:
“Does my Imaging system accurately measure
...with high enoughresolution?
… in high gradients?
Where are the LSF, PSF, or MTF?
You should not assume it will be correct just
because I paid a lot of money for it!
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